This invention relates to control of a fuel supply to a combustor of a gas turbine engine.
Under some conditions, the flame in a gas turbine engine combustor may be extinguished unintentionally. This is known as flame out. The present invention addresses the problem of flame out occurring when fuel flow is reduced to decelerate an engine rapidly from a high power setting to idle.
Engine deceleration is achieved by reducing the flow of fuel to the combustors of the engine and, if the fuel flow reduction is not carefully controlled, the fuel air ratio (FAR) may fall to a level at which the flame can no longer sustain itself and is extinguished. However, a high rate of engine deceleration and consequent thrust loss is desirable in many situations, for example during combat. As a result the control system must set minimum fuel flow according to a trade off between maximising the rate of deceleration and minimising the risk of flame out.
In practice, a stability margin is set to minimise the possibility of flame out. That stability margin takes account of the FAR weak extinction threshold but also includes factors which take account of control logic sensitivity to ambient temperature, differences between engine builds, control system tolerances and other such possible errors. The stable control of an engine requires the supply of fuel to be limited to keep the FAR below the stability margin.
It is known to base the fuel flow rate to a gas turbine engine combustor on, inter alia, the square root of non-dimensional engine entry temperature (theta). Theta is the result of a division of the engine inlet temperature (T1) by the international standard atmospheric temperature (TISA). While using the square root of theta can ensure that engine operation is controlled as close as possible to a fixed stability margin that maximises the rate of deceleration for one fixed ambient temperature, this method exhibits sensitivity to ambient temperature. The result is that during cold days, the rate of deceleration may be limited more than necessary to account for a higher likelihood of flame out with the same engine conditions during a hot day. The present invention aims to eliminate the need for additional FAR margin to be used to accommodate changes in ambient temperature by using a FAR calculation that is insensitive to ambient temperature.